Lupus nephritis (LN) leads to renal failure in up to 50% of cases over 5 years despite aggressive immune suppressing therapies that are nonspecific and often contribute to significant morbidity and mortality. Further investigations into the pathogenesis of LN are necessary to develop targeted therapies. Elevated nitric oxide (NO), isoprostane, and thromboxane (TXA2) and reduced prostacyclin (PGI2) synthesis occur in LN. TXA2 synthase inhibitors, TXA2 receptor antagonists, and inducible NO synthase (iNOS) inhibitors abrogate murine LN, while the same TXA2 specific therapies improve renal function in human LN. No combines with superoxide to form peroxynitrite (ONOO), which can alter TXA2 and PGI2 metabolism. Thus, one mechanism by which NO may be pathogenic in LN is through the effect of ONOO-induced alternations in intra renal accessioned synthesis. To test this hypothesis, the following specific aims are proposed: 1) Correlate systemic ONOO production in humans with LN with urinary excretion of 8-is PGF2a ( an isoprostane) and the excreted renal metabolites of TXA2, PGI2 and PGE2. Alter ONOO production in murine LN using pharmacologic an genetic means and measure the renal synthesis and urinary excretion of the same eicosanoids, GFR, and nitration of renal TXA2 and PGI2 synthase. 3) Isolate glomeruli from murine models of LN and measure directly the effects of exogenous NO and ONOO donors and iNOS inhibitors on accessioned synthesis and determine if nitration of renal TXA2 and PGI2 synthase occurs. We will measure longitudinal SLE and LN disease activity (in 38 SLG and 38 LN subjects compared to 38 controls) and correlated these with serum 3NT and renal accessioned synthesis. To determine if NO and ONOO exert downstream effects on accessioned metabolism, we will measure the same parameters in 2 models of murine LN in response to iNOS inhibition (with iNOS inhibitors, vitamin E, and MRL/lpr iNOS -/- mice). Glomeruli from murine models of LN will be isolated to determine in vitro the effect of iNOS inhibitors and NO/ONOO- donors on accessioned synthesis. Determining the pathogenic mechanisms of ONOO production as it relates to accessioned synthesis in LN may lead to the use of specific iNOS inhibitors in humans with LN. If we disprove our hypothesis that ONOO induces alternations in eicosanoid synthesis in LN iNOS activity and eicosanoids synthesis may be triggered by common or parallel upstream mechanisms. Alternatively, eicosanoids could regulate NO synthesis in LN as they can in some experimental systems. In either event, future studies would address the use of selective COX-2 or TXA2 synthase inhibitors/receptor blockers, antioxidants, and/or PGI2/PGE2 agonists in conduction with iNOS inhibitors as therapies for LN.